Nonvolatile memory elements are used in systems in which persistent storage is required. For example, digital cameras use nonvolatile memory cards to store images and digital music players use nonvolatile memory to store audio data. Nonvolatile memory is also used to persistently store data in computer environments. Nonvolatile memory is often formed using electrically-erasable programmable read only memory (EPROM) technology. This type of nonvolatile memory contains floating gate transistors that can be selectively programmed or erased by application of suitable voltages to their terminals.
As fabrication techniques improve, it is becoming possible to fabricate nonvolatile memory elements with increasingly smaller dimensions. However, as device dimensions shrink, scaling issues are posing challenges for traditional nonvolatile memory technology. This has led to the investigation of alternative nonvolatile memory technologies, including resistive switching nonvolatile memory.
Resistive switching nonvolatile random access memory (ReRAM) is formed using memory elements that have two or more stable states with different resistances. Bistable memory has two stable states. A bistable memory element can be placed in a high resistance state or a low resistance state by application of suitable voltages or currents. Voltage pulses are typically used to switch the memory element from one resistance state to the other. Nondestructive read operations can be performed to ascertain the value of a data bit that is stored in a memory cell.
Resistive switching based on transition metal oxide switching elements formed of metal oxide films has been demonstrated. Although metal oxide films such as these exhibit bistability, the resistance of these films and the ratio of the high-to-low resistance states are often insufficient to be of use within a practical nonvolatile memory device. For instance, the resistance states of the metal oxide film should preferably be significant as compared to that of the system (e.g., the memory device and associated circuitry) so that any change in the resistance state change is perceptible. The variation of the difference in resistive states is related to the resistance of the resistive switching layer. Therefore, a low resistance metal oxide film may not form a reliable nonvolatile memory device. For example, in a nonvolatile memory that has conductive lines formed of a relatively high resistance metal such as tungsten, the resistance of the conductive lines may overwhelm the resistance of the metal oxide resistive switching element. Therefore, the state of the bistable metal oxide resistive switching element may be difficult or impossible to sense.
Current ReRAM structures use multi-layer thin film stacks of insulator materials, such as metal oxides (e.g., hafnium oxide or zirconium oxide) between metal electrodes to form a device that can be switched between two different stable resistance states by the application of appropriate voltages. A capping layer can be provided on the top electrode to improve the thin film stack fabrication or the ReRAM operation. For example, a capping layer can protect the electrode from being oxidized or from being etched during a plasma stripping or a plasma etch process. In some cases, the capping layer might increase the conductivity for interconnect lines connecting adjacent ReRAM devices. The capping layer can also serve as a gettering agent during device fabrication or operation. For example, a titanium capping layer can attract oxygen from the metal oxide layer, helping oxygen vacancies for form more easily, thus reducing the setting voltage.
The manufacture of multi-layer ReRAM structures, and similar semiconductor devices typically involves patterning of the multiple layers by etching using liquid or wet etching solutions. For example, one well known etching solution is hydrofluoric acid, which can etch the silicon oxide isotropically, i.e., same etch rate in both lateral and perpendicular directions. This can result in etched features that have smaller line widths than those of the patterning images.
Patterning using etching can also use dry etching techniques, which are conducted in a gas phase using physical sputtering by target bombardment with energetic particles such as argon, or using plasma etching by reactive ions of halogens or halogen-containing compounds. The dry etching technology employs exited ions, typically generated by an RF plasma, for etching the layer materials. In the dry etch process, etching is typically anisotropic, resulting in etched features that have line widths substantially matching those of the patterning images. Generally, the term “plasma etching” describes reactive ion etching, where ions of reactive elements (such as halogen elements of fluorine, chlorine, etc.) are excited by a plasma, and then allowed to react with the layer materials to form volatile species. The terms “physical sputtering” and “physical sputter etch” are used interchangeably in the present description, and describe bombardment of the substrate with energetic particles which can be generated by a plasma or an ion source. The sputtering process is driven by momentum exchange between the energetic particles and the layer materials, and therefore the particle in a physical sputtering process is typically an inert element, such as argon or xenon.
An important aspect in forming a multi-layer semiconductor device is the control of the patterning process with respect to the different layers in the metal oxide stack. The poor selectivity can cause under or over etch issues and cross-contamination/residues remaining after the etch process.
Therefore, a need exists for a process for high selective etching of multi-layer substrates.